You sound like a great neighbour! I just know that couldn't be me. 

I'm too petty with petty people! 

 but honestly, I would leave it. If they keep complaining, and you obviously want to keep things neighbourly, just tell them that unfortunately you have tried to fix it to no avail, and you can swing some eggs their way occasionally to keep everyone happy. But honestly, I think they need to grow up & get a hobby. I've had neighbours from hell & they only got worse until I threatened the cops as I had video footage of them beating their dogs (after threatening me with RSPCA for my dogs being outside). Now I wouldn't say threatening them is the answer, but I personally would leave it, ignore all complaints & advise them to take it up with council & waste someone else's time. 

 good luck! What a pain having to deal with that.